It can be anything! An apex, subdomain, or even a URL.

Results formail.google.com

Resolver

Location

A

Name	TTL	Value
mail.google.com	300	2607:f8b0:4004:c1b::11Google Public DNS (iad) / Google LLC
mail.google.com	300	2607:f8b0:4004:c1b::12Google Public DNS (iad) / Google LLC
mail.google.com	300	2607:f8b0:4004:c1b::13Google Public DNS (iad) / Google LLC
mail.google.com	300	2607:f8b0:4004:c1b::53Google Public DNS (iad) / Google LLC

Name	TTL	Value
mail.google.com	86400	google-site-verification=PncXpRKRCAlDAdlesTtNFf6k9TvgxgcRfojdaKkEACY